How long does it take to train to become a medical transcriptionist?

It depends on the training (and/or certification - if required by the company) that you are going to take for this field. If you take it via training centers or via the training room for the company you work for (since most MT companies offer free medical transcription training to newbies in the field), then the duration of the training will have to be based on the schedules that you have. If you will take up online training programs for them, since most of them are self paced, then it would be up to you on how long it would take you to finish up the course. Most online courses would last 390 to 420 training hours.